1. Field of the Invention
The present invention relates to an apparatus for pneumatic length measurement comprising a pre-nozzle and a measurement nozzle through which the medium used for the measurement is directed onto an impact plate, a first pressure sensor arranged in front of the pre-nozzle, a second pressure sensor arranged between pre-nozzle and measurement nozzle, means for the determination of the distance of the impact plate from the measurement nozzle from the change of the pressure between pre-nozzle and measurement nozzle taking account of the pressure measured by the first pressure sensor and means for the calibration of the apparatus in a given measurement range.
2. Description of the Prior Art
An apparatus of this kind is known from DE 197 33 984.0. There, an apparatus is described in conjunction with a pneumatic high pressure length measuring method which operates with only two nozzles instead of the known bridge circuits. Through the use of a pressure sensor in front of the pre-nozzle it is possible to dispense with the two further nozzles of the reference branch in the bridge circuit. The air consumption associated with the reference branch is thus avoided.
For the calibration of the known apparatus various calibration formulae are named in DE 197 33 984.0. All require a value measurement at two different fixed lengths. That is to say, two setting masters are required in order to calibrate the apparatus and to determine the measurement function.
The present invention is based on the object of setting forth an apparatus of the initially named kind which is easier to calibrate.
This object is satisfied in that the calibration means operates with a calibration function which is constructed on the assumption of constancy of the outflow number of the pre-nozzle and of the outflow number of the measurement nozzle.
Through the use of such a calibration function the calibration is simplified. The use of a calibration function is which is additionally constructed on the assumption of constant diameter of the pre-nozzle and of the measurement nozzle is particularly preferred. In this way a calibration function can be used with only one constant for the calibration of the apparatus. Since only one constant has to be determined, the measurement at one fixed preset length value is sufficient for the calibration. That is to say, only one setting master is required in order to carry out the calibration. Accordingly, the calibration for the apparatus of the invention can thus take place simply and rapidly.
A calibration function of this kind, that is to say the calibration method used in the initially described apparatus, can also be used in all pneumatic length measurement apparatuses, and thus also in length measurement apparatuses which operate with a bridge circuit, or also with low pressure length measurement apparatuses. In this connection the invention is based on the recognition that the supply pressure and the temperature are essentially constant when using these apparatuses, and thus also the outflow numbers and the diameter of the pre-nozzle and measurement nozzle, so that a calibration function constructed under these assumptions, which is correspondingly simple, delivers usable results.
In accordance with a design of the invention the apparatus is designed for high pressure measurement methods, i.e. supply pressures greater than 2 bar, in particular circa 3 bar. In this respect it has proved to be suitable when the calibration means use the function   x  =      C    ⁢                  p        1                    p        2              ⁢                                        (                                          p                2                                            p                1                                      )                                2            κ                          -                              (                                          p                2                                            p                1                                      )                                              κ              +              1                        κ                              
as the calibration function, with xcexa being the adiabatic constant of the medium used for the measurement and C being a constant. The constant C can easily be determined with only one setting master of the length X2 in accordance with the following formulae:   C  =            X      2        ⁢                  P        2                              P          1                ⁢                                                            (                                                      P                    2                                                        P                    1                                                  )                                            1                ,                42857                                      -                                          (                                                      P                    2                                                        P                    1                                                  )                                            1                ,                71429                                                        
with P1 being the magnitude of the supply pressure p1 during calibration and P2 being the pressure p2 measured with the inserted setting master X2. As has been shown, very small deviations over the full measurement range can be achieved with this calibration function. The calibrated measurement range can indeed be enlarged in comparison to apparatuses of which the calibration means operate with known calibration functions. Moreover, measurement nozzles of different diameters can be used advantageously, since the sensitivity is automatically co-calibrated.
In accordance with a further design of the invention, the calibration means can use the function:   x  =                                          p            1                                p            2                          -        1            B      
as the calibration function, with B being a constant. This is a relatively simple calibration function which exploits the non-linear relationship between p2 and p1. The constant B can be determined as follows with a single setting master X2:   B  =            1              X        2        2              ⁢          (                                    P            1                                P            2                          -        1            )      
with P1 again being the magnitude of the supply pressure p1 during calibration and P2 again being the measured pressure p2 with the inserted setting master X2. This design also has the advantage of a large calibrateable range of measurement. Moreover, the ability to use measurement nozzles with different diameters also applies here, since the sensitivity is automatically co-calibrated.